This invention relates to large area semiconductor junction devices for use as photocells, and as solar cells in particular.
The national concern over energy and energy sources has generated intense interest in the potential of solar power. Conversion of solar energy into electricity is expensive. The conversion devices most often considered are solar batteries, which are semiconductor devices. For conventional electrical applications, such as switching or signal amplification, semiconductor devices are extremely inexpensive because they can be made very small. The cost of semiconductor devices is mainly dependant on the size of the device. Since solar batteries collect light in proportion to the area of the photoresponsive junction, which needs to be relatively large to generate a useful amount of photocurrent, they are costly power generators.
Considerable effort and expense have been devoted to finding ways to reduce the cost of semiconductor solar devices. Much of this has been expended in attempts to make devices in which the semiconductor material is deposited as a polycrystalline thin film on an inexpensive substrate rather than grown by a costly single crystal technique. An example of this approach is described in U.S. Pat. No. 3,953,876 issued Apr. 27, 1976. As in this example, much of the prior effort has been directed to making silicon devices, the most conventional solar cell.
A form of solar cell that has generated enthusiasm recently is the liquid-semiconductor junction solar cell. As implied by the name, the active part of these devices is a junction formed at a semiconductorliquid interface. This kind of device promises to be less expensive to manufacture because it does not require relatively costly epitaxy or diffusion procedures to form the junction, but rather the junction forms spontaneously at the liquid-solid interface. However, the need for a single crystal semiconductor body so far remains.
Significant advances have occurred recently in liquid-semiconductor junction solar cells. These devices were reviewed in a paper written in 1974 by Heinz Gerischer, Electroanalytical Chemistry and Interfacial Electrochemistry, 58 (1975), 263-274. His paper deals largely with junctions based on CdS, chosen because of the position of the Fermi level of CdS and because in this kind of cell CdS shows more promise of being photochemically stable than silicon, a material with a more favorable bandgap. Gerischer also considered cells based on CdSe, which has a favorable bandgap, and is potentially more efficient therefore than CdS in terms of solar power conversion. Devices using GaP were also considered but these gave considerably less power. The results reported by Gerischer do not establish the intrinsic merits of these semiconductors in solar devices because they are based on one particular liquid redox system. The choice of the liquid that forms part of the junction determines the photochemistry of the device. Gerischer reported on the ferrocyanide-ferricyanide redox electrolyte which he found to be corrosive to CdS. It is known that this liquid-semiconductor junction is not photochemically stable. Photoexcitation which produces holes at the CdS surface tends to corrode CdS, producing a sulfur layer at the junction interface. This photocorrosion mechanism is manifested by a continuous decay of the photocurrent from the cell with operating time. The reaction is: CdS + 2h.sup.+ + redox electrolyte .fwdarw. S + Cd.sup.2+. The use of a redox couple that functions at a potential higher than this, namely the ferro-ferricyanide couple of Gerischer et al, will also produce a corrosion reaction. The problem of corrosion and consequent passivation of the semiconductor element of the junction is a serious obstacle to practical liquid-semiconductor junction solar cells.
Attempts to repress the corrosion reaction in the presence of other redox couples by increasing Cd.sup.2+ and S concentration in the liquid electrode are limited in application.
Another semiconductor, n-type TiO.sub.2, was found by Fujishima and Honda, Nature, 238 (1972) 37, to be electrochemically stable in certain aqueous electrolytes. However, as Gerischer states, "This material is however excluded from a practical application for solar energy conversion by its large bandgap." Moreover, in the system investigated by Fujishima et al, in which the semiconductor is stable, the electrolyte itself is consumed, producing hydrogen, and is therefore not suitable for photoelectric energy conversion. Other semiconductors have been found recently to be appropriately stable in these kinds of cells, but all have large bandgaps.
One way of overcoming the corrosion problem occurs readily to a skilled electrochemist and indeed was proposed to Gerischer in 1966 by G. C. Barker, J. Electrochem. Society, 113 (1966) 1182. The solution is to use a polysulfide-sulfide redox couple as the liquid electrode with a CdS solid electrode.
The corrosion reaction: CdS + 2h.sup.+ .fwdarw. Cd.sup.++ +S proceeds at a higher (more anodic) electrode potential than S.sup.- .fwdarw. S + 2e. Therefore, the sulfur-polysulfide couple (and others with comparable redox potentials) consume the holes responsible for the corrosion reaction before the potential for that reaction is reached. We regard this as a fundamental principle of electrochemistry and have amply demonstrated its effectiveness in liquidchalcogenide semiconductor solar cells. The other chalcogenides, selenide and telluride, follow the same principle, i.e., they are stable in redox electrolytes containing selenide or telluride as the reduced form of the couple. We have found cases (e.g., CdSe in sulfide) of effective operation where the anion of the solid semiconductor and that of the electrolyte do not match, leading to the conclusion that for a given chalcogenide semiconductor the reduced form of the redox couple may be sulfur, selenium or tellurium, or even mixtures of these anions.
The solution of the corrosion problem leaves one major obstacle to practical use of solar batteries as competitive power sources. That obstacle, which was pointed out earlier in connection with solid state junction solar cells, is the cost of single crystal semiconductor material.